• Idiopathic guttate hypomelanosis (IGH) has been studied histochemically (split-dopa) and ultrastructurally in three patients. Affected skin was compared to normal surrounding skin. Argentic stains revealed a decrease in the melanin content of affected epidermis and pigment granules were irregularly distributed. Split-dopa preparations showed a decrease in the number of dopa-positive melanocytes in hypomelanotic macules. Most of the melanocytes were rounded in form and either lacked dendrites or had fragmented ones. Ultrastructural study confirmed the progressive loss of epidermal melanocytes in skin affected by IGH and revealed two types of melanocytes, healthy melanocytes with normal melanogenic activity and melanocytes containing few or no immature melanosomes but showing no cellular alterations. Depigmentation in IGH seems to occur in two stages, loss of melanogenic activity in certain melanocytes and elimination of inactive melanocytes. The basic defect in IGH melanocytes may reflect the aging process.